Fluid ejection heads are useful for ejecting a variety of fluids including inks, cooling fluids, pharmaceuticals, lubricants and the like. A widely used fluid ejection head is in an ink jet printer. However, fluid ejection heads may also be used in vaporization devices for vapor therapy, E-cigarettes and the like. New techniques are constantly being developed to provide low cost, highly reliable fluid ejection heads for such devices.
The fluid ejection head is a seemingly simple device that has a relatively complicated structure containing electrical circuits, ink passageways and a variety of tiny parts assembled with precision to provide a powerful, yet versatile fluid ejection head. The components of the ejection head must cooperate with each other and be useful for a variety of fluids and fluid formulations. Accordingly, it is important to match the ejection head components to the fluid being ejected. Slight variations in production quality can have a tremendous influence on the product yield and resulting ejection head performance.
The primary components of a fluid ejection head are a semiconductor substrate, a flow feature layer, a nozzle plate layer, and a flexible circuit attached to the substrate. The semiconductor substrate is preferably made of silicon and contains various passivation layers, conductive metal layers, resistive layers, insulative layers and protective layers deposited on a device surface thereof. Fluid ejection actuators formed on a device surface of the substrate may be thermal actuators or piezoelectric actuators. For thermal actuators, individual heater resistors are defined in the resistive layers and each heater resistor corresponds to a nozzle hole in the nozzle plate for heating and ejecting fluid from the ejection head toward a desired substrate or target.
The nozzle plate layer and the flow feature layer may each be made of a photoimageable material. The flow feature layer is typically spin-coated onto the substrate, imaged and developed to provide flow features therein for directing fluid from a fluid via in the substrate to a heater on the substrate. Next, the substrate is etched to form the fluid vias in a dry reactive ion etching (DRIE) process. A dry film photoresist nozzle layer is then laminated to the flow feature layer using heated, pressure rollers. Finally, the nozzle layer is exposed and developed to form the nozzle holes therein.
During the nozzle plate layer lamination process, the nozzle plate layer is bridged across the fluid via in the substrate and the flow features in the flow feature layer. Stresses are introduced from the laminator rollers into the nozzle plate layer as the laminator roll radially compresses the nozzle plate layer, pushing the nozzle plate layer down into unsupported regions of the flow feature layer. The radial compression of the nozzle plate layer causes UV light diffraction during nozzle plate layer imaging, resulting in poor nozzle hole image quality. A second factor contributing to poor nozzle hole image quality is the fact that the heaters on the substrate are constructed of highly reflective material. The highly reflective material causes UV light rays used during nozzle plate imaging to reflect back into the nozzle plate layer photoresist material causing areas of the unexposed nozzle holes to be exposed to small amounts of UV radiation that results in misshapen nozzle holes.
The planarity of the nozzle plate layer is critical to controlling fluid ejection directionality. Nozzle hole diameter variances also may cause drop mass inconsistencies. Accordingly, there continues to be a need for manufacturing processes and techniques which provide improved fluid ejection head components.
In view of the foregoing, embodiments of the disclosure provide improved fluid ejection heads and a method for making fluid ejection heads. The method includes the steps of: providing a semiconductor substrate containing a plurality fluid ejection actuators on a device surface thereof; reactive ion etching of the substrate to form one or more fluid supply vias therein; laminating a flow feature layer to a device surface of the substrate; exposing the flow feature layer to ultra violet (UV) radiation through a photo mask to provide UV exposed areas of the flow feature layer; heating the flow feature layer to cross-link material in the UV exposed areas of the flow feature layer; laminating a nozzle plate layer to the flow feature layer; exposing the nozzle plate layer to UV radiation through a photo mask to provide UV exposed areas for nozzle holes; cross-linking the nozzle plate layer with heat; and developing the flow feature layer and nozzle plate layer to form the flow features and nozzle holes in the respective layers.
In another embodiment, there is provided a fluid ejection head. The fluid ejection head includes a semiconductor substrate containing a plurality fluid ejection actuators on a device surface thereof and one or more fluid supply vias etched therethrough; a first photoresist layer for flow features laminated to the device surface of the semiconductor substrate; and a second photoresist layer for nozzle holes laminated to the first photoresist layer. The second photoresist layer includes (a) hydrophilic photoresist material layer disposed adjacent to the flow feature layer and (b) a hydrophobic photoresist material layer disposed adjacent to (a).
In one embodiment, the flow feature layer provides an anti-reflective coating on the substrate. In another embodiment, the flow feature layer comprises a negative photoresist material with a thickness ranging from about 5 to about 50 μm, such as from about 10 to about 30 μm.
In another embodiment, the nozzle plate layer comprises a negative photoresist material having a thickness ranging from about 5 to about 50 μm, such as from about 10 to about 30 μm.
In some embodiments, the nozzle plate layer includes a combination of (a) hydrophilic photoresist material layer disposed adjacent to the flow feature layer and (b) a hydrophobic photoresist material layer disposed adjacent to (a), wherein layer (a) has a water contact angle ranging from about 60 to less than about 90 degrees, and layer (b) has a water contact angle ranging from about 91 to about 120 degrees.
In some embodiments, the device surface of the substrate is plasma treated for promoting adhesion between the flow feature layer and the device surface of the substrate, and the flow feature layer is plasma treated with oxygen and a forming gas to promote adhesion between the flow feature layer and the nozzle plate layer.
In some embodiments, the fluid ejection head comprises an inkjet printhead.
An advantage of the embodiments described herein is that the first photoresist layer, or flow feature layer, is effective to minimize stresses on the second photoresist layer, or nozzle plate layer, during a lamination process for the second photoresist layer. Another advantage of the disclosed embodiments, is that the first photoresist layer may be used as an anti-reflective layer between the second photoresist layer and highly reflective heater surfaces during an imaging step for the second photoresist layer. Yet another advantage of the disclosed embodiments is that a reactive ion etching process for fluid flow vias in the substrate is conducted before the first photoresist layer is applied to the substrate thereby avoiding flow feature damage during the etching or striping processes, which would enable more aggressive strip processes for higher ejection head yields.